If you're talking about AirPlay Mirror stream from a computer to Apple TV, this is the reason why I returned my Apple TV unit...it simply does not stream perfectly for AirPlay mirroring (when trying to play legitimate film files in anything other than low to standard definition). Blocky, choppy and laggy performance is what I experienced 90% of the time, and it wasn't up to par with what I'm used to using other methods.

In this case, ISP really isn't as relevant as network hardware...I'm using a dual band NetGear router setup with signal booster, and having had no issues with previous units doing the same thing, I could definitely tell it was the Apple TV's mirroring function that was falling short. Actually tried two units too, and had the same results.